Maintainability is defined as the measure of the ability of an item to be retained in, or restored to specified condition when maintenance is performed by personnel having specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair.

This is directly analogous to repairability. The difference is merely that maintainability is based on total downtime (which includes active repair time, logistic time, and  administrative time), while repairability is restricted solely to active repair time.

The analogy holds with respect to the associated functions as well. The maintainability function is the cumulative probability that the failed system is restored to operable condition in not more than a specified downtime, expressed as a function of this downtime. The corresponding density function is called the maintenance time density function.

References:

1. MIL-HDBK-338, Electronic Reliability Design Handbook, 15 Oct 84
2. MIL-STD-721, Definitions of Terms for Reliability and Maintainability, 23 Oct 91